When shoes, in particular sports shoes, are manufactured, two objectives are to provide a good grip on the ground and to sufficiently cushion the ground reaction forces arising during the step cycle, in order to reduce strain on the muscles and the bones. In traditional shoe manufacturing, the first objective is addressed by the outsole; whereas, for cushioning, a midsole is typically arranged above the outsole. In shoes subjected to greater mechanical loads, the midsole is typically manufactured from continuously foamed ethylene vinyl acetate (EVA).
Detailed research of the biomechanics of a foot during running has shown, however, that a homogeneously shaped midsole is not well suited for the complex processes occurring during the step cycle. The course of motion from ground contact with the heel until push-off with the toe part is a three-dimensional process including a multitude of complex rotating movements of the foot from the lateral side to the medial side and back.
To better control this course of motion, separate cushioning elements have, in the past, been arranged in certain parts of the midsole. The separate cushioning elements selectively influence the course of motion during the various phases of the step cycle. An example of such a sole construction is found in German Patent No. DE 101 12 821, the disclosure of which is hereby incorporated herein by reference in its entirety. The heel area of the shoe disclosed in that document includes several separate deformation elements having different degrees of hardness. During ground contact with the heel, the deformation elements bring the foot into a correct position for the subsequent rolling-off and pushing-off phases. Typically, the deformation elements are made from foamed materials such as EVA or polyurethane (PU).
Although foamed materials are generally well suited for use in midsoles, it has been found that they cause considerable problems in certain situations. For example, a general shortcoming, and a particular disadvantage for running shoes, is the comparatively high weight of the dense foams.
A further disadvantage is the low temperature properties of the foamed materials. One may run or jog during every season of the year. However, the elastic recovery of foamed materials decreases substantially at temperatures below freezing, as exemplified by the dashed line in the hysteresis graph of FIG. 6C, which depicts the compression behavior of a foamed deformation element at −25° C. As can be seen, the foamed deformation element loses to a great extent its elastic recovery and, as represented by the arrow 9 in FIG. 6C, partly remains in a compressed state even after the external force has been completely removed. Similar effects, as well as an accelerated wear of the foamed materials, are also observed at higher temperatures.
Additionally, where foamed materials are used, the ability to achieve certain deformation properties is very limited. The thickness of the foamed materials is, typically, determined by the dimensions of the shoe sole and is not, therefore, variable. As such, the type of foamed material used is the only parameter that may be varied to yield a softer or harder cushioning, as desired.
Accordingly, foamed materials in the midsole have, in some cases, been replaced by other elastically deformable structures. For example, U.S. Pat. Nos. 4,611,412 and 4,753,021, the disclosures of which are hereby incorporated herein by reference in their entirety, disclose ribs that run in parallel. The ribs are optionally interconnected by elastic bridging elements. The bridging elements are thinner than the ribs themselves so that they may be elastically stretched when the ribs are deflected. Further examples may be found in European Patents Nos. EP 0 558 541, EP 0 694 264, and EP 0 741 529, U.S. Pat. Nos. 5,461,800 and 5,822,886, and U.S. Des. Pat. No. 376,471, all the disclosures of which are also hereby incorporated herein by reference in their entirety.
These constructions for the replacement of the foamed materials are not, however, generally accepted. They do not, for instance, demonstrate the advantageous properties of foamed materials at normal temperatures, such as, for example, good cushioning, comfort for the wearer resulting therefrom, and durability.
It is, therefore, an object of the present invention to provide a shoe sole that overcomes both the disadvantages present in shoe soles having foamed materials and the disadvantages present in shoe soles having other elastically deformable structures.